MESA, Ariz. -- Cubs pitcher Matt Garza, sidelined with a strained left lat, is making progress and even able to laugh about what happened, but only a little.

Garza ended a scheduled 40-pitch throwing session Sunday after 20 pitches because of discomfort in his left side. He is getting treatment and has yet to resume baseball activities.

"Right now, I just want to get ready to throw, get ready to start pitching for opening week, and that's where my timeline's at," said Garza, a candidate for Opening Day along with Jeff Samardzija and Edwin Jackson. "Spring starts -- of course, they matter -- but I'm more concerned about being ready by Opening Day."

Garza arrived in camp feeling strong after missing more than two months last season because of a sore right elbow, which shut him down after his July 21 start.

"Maybe that's what it was -- maybe I came out too strong, too fast," he said Thursday. "It is what it is, and now it's time to get back to work."

Spring Training camps opened a few days early so players could prepare for the World Baseball Classic, and the extra time may help Garza.

"It's like [Dr. Stephen Adams] said, 'Hey, time didn't hurt anybody, so just relax, enjoy it a little bit, because this will be the last time you'll be off,'" Garza said. "That's kind of the plan."

It doesn't hurt to raise his left arm.

"It's just slow," Garza said of his recovery. "Those muscles are the ones you have to really be careful with because they're involved in everything you do. Some people say I shouldn't even be laughing, but it doesn't hurt, so that's the good thing."